PRODUCT DATASHEET
AAT1217
SwitchReg
TM
Current Mode Operation
The AAT1217 is based on a slope compensated current
mode PWM control topology. It operates at a fixed fre-
quency of 1.2MHz. At the beginning of each clock cycle,
the main switch (NMOS) is turned on and the inductor
current starts to ramp. After the maximum duty cycle or
the sense current signal equals the error amplifier (EA)
output, the main switch is turned off and the synchro-
nous switch (PMOS) is turned on. This control topology
features cycle-by-cycle current limiting which can pre-
vent the main switch from overstress and the external
inductor from saturating.
600mA, 1.2MHz, Micropower Synchronous Step-Up Converter
Application Information
Adjustable Output Voltage
An external resistor divider is used to set the output volt-
age. The output voltage of the switching regulator (V
OUT
)
is determined by the following equation:
R1
⎞
⎛
V
OUT
= 1.23V · 1 + R2
⎝
⎠
Table 1 lists the recommended resistor values for par-
ticular output voltage settings.
V
OUT
3.3V
5.0V
Light Load Mode Operation
At very light load, the AAT1217 automatically switches
into light load mode operation to improve efficiency.
During this mode, the PWM control will skip some pulses
to maintain regulation. If the load increases and the out-
put voltage drops, the device will automatically switch
back to normal PWM mode and maintain regulation.
R1(Ω)
1.02M
1.02M
R2(Ω)
604k
332k
Table 1: Resistor Selection for
Output Voltage Setting.
Fixed Output Voltage
The AAT1217 has two fixed output voltage options: 3.3V
and 5V. An internal resistor divider is connected to the
FB pin inside the package which eliminates the need for
external feedback resistors. When designing with the
fixed output voltage option, remember to leave the FB
pin open; otherwise the output voltage will be affected.
However, a feed-forward capacitor can still be added
between the FB and VOUT pins to enhance the control
loop performance.
Anti-Ringing Control
An anti-ringing circuitry is included to remove the high
frequency ringing that appears on the SW pin when the
inductor current goes to zero. In this case, a ringing on
the SW pin is induced due to remaining energy stored in
parasitic components of switch and inductor. The anti-
ringing circuitry clamps the voltage internally to the bat-
tery voltage and therefore dampens this ringing.
Device Shutdown
When SHDN is set logic high, the AAT1217 is put into
active mode operation. If SHDN is set logic low, the
device is put into shutdown mode and consumes less
than 1μA of current. After start-up, the internal circuitry
is supplied by V
OUT
, however, if shutdown mode is
enabled, the internal circuitry will be supplied by the
input source again.
Inductor Selection
The high switching frequency of 1.2MHz allows for small
surface mount inductors. For most applications, the
AAT1217 operates with inductors from 2.2μH to 10μH.
Use the following equations to select the proper inductor
value for a particular application condition:
ΔI
L
I
PEAK
= I
DC
+ 2
I
DC
=
ΔI
L
=
ΔI
L
=
I
OUT
· V
OUT
η
· V
IN
V
IN
· D
V
OUT
- V
IN
; D=
L · F
SW
V
OUT
V
IN
· (V
OUT
- V
IN
)
L · F
SW
· V
OUT
1217.2008.03.1.3
8
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